1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms an image by developing an electrostatic latent image formed on an image carrier with a toner.
2. Description of the Related Art
An image forming apparatus, such as a dry copying machine, frequently uses a developing device that develops an electrostatic latent image on a photosensitive drum by using a two-component developer made up of toner and a carrier. In such an image forming apparatus, the toner density (the ratio of the toner to the carrier) of the two-component developer is detected with a toner density sensor for detecting the toner density of the developer within the developing device. Toner density may be controlled by replenishing the toner on the basis of the detected ratio, thereby keeping the toner density constant.
The assembly in which the toner and carrier are mixed and stirred and the image on the photosensitive drum surface is developed is often provided as an assembled unit replaceable by the end user or a service engineer because of its high susceptibility to wear. The toner to be consumed is also modularized into a toner cartridge readily replaceable by the end user. Since the toner cartridge predictably runs out of toner during the life cycle of the developing device, the toner cartridge is typically replaced a number of times during the service life of the developing device.
When residual toner collected in the cleaning process of the photosensitive drum is returned into the developing device for reuse, the toner collected inevitably includes paper dust contaminants, which end up in the developing device. If the quantity of the collected toner and paper dust in the developing device increases beyond a prescribed level, it will affect the electric charge and toner grain size distribution in the developer within the developing device. Eventually, this will invite problems such as intensified fogging of the background due to a low charge and smearing caused by scattered toner.
The quantity of toner collected in the developing device is correlated to the amount of toner supplied from the toner cartridge. The more frequently the toner cartridge is replaced, the higher the proportion of toner collected in the developing device, leading to the problems noted above.
In an apparatus that detects the amount of toner remaining in the toner cartridge using a toner density detecting sensor, the emptiness of the toner cartridge is detected from a drop in toner density within the developing device. In such a case, a temporary drop in toner density within the developing device at the time the amount of toner remaining in the toner cartridge is detected invites deterioration of the developer within the developing device. After the next toner cartridge replacement, if the output level of the toner detecting sensor is controlled to the control target value, the deterioration of the developer will raise the toner density, eventually leading to a problem of an increased density of the output image.
Regarding this problem, an apparatus equipped with a density sensor that directly measures the density of the toner on the photosensitive drum could control the toner density to prevent the output image from becoming too dense. However, in an apparatus having no such density sensor, a rise in toner density would also make the output image too dense, resulting in an increase in toner consumption and, accordingly, a rise in cost.
Thus, as shown in FIGS. 5A, 5B and 5C, even though the replacement of the toner cartridge is repeated, if an increasing number of sheets are printed with the target control value of the toner density detecting sensor kept fixed as the toner density (FIG. 5A), the quantity of collected toner and paper dust will increase with the number of sheets printed. Further, after the replacement of a toner cartridge having run out of toner, the toner density that has temporarily fallen is recovered to the control target value under the control of the toner detecting sensor (FIG. 5B), but the deterioration of the developer due to the toner density drop will eventually lead to a denser image as shown in FIG. 5C.
This problem naturally compounds as the toner cartridge replacement is repeated. The resulting undesirable increase in image density may intensify with the frequency of toner cartridge replacements.
To address this problem, it has been proposed to provide toner cartridges in which the characteristics of the toner contained in the toner cartridges is different from one cartridge to another, so that the replacement of the toner cartridge may not result in an increased image density. However, this arrangement requires installation of plural toner cartridges in a single apparatus, which may lead to confusion on the user's part or a rise in the manufacturing and storage costs of the toner and toner cartridges.